fn nth3<A: Clone>(v: &Vec<A>, n: usize) -> Option<&A> {
    match v.first() {
        None => None,
        Some(elm) =>
         if n == 0 { Some(elm) } else {
             let (_, sl) = v.split_at(1);
             let vv = sl.to_vec();
             nth3(&vv, n - 1)
         }
    }
}